It is important to the manufacture of trimellitic acid to have at least the cobalt, the most expensive catalyst metal, recycled to the pseudocumene oxidation. For most catalytic liquid phase oxidations of methylbenzenes to benzene carboxylic acids the use of both cobalt and manganese with an oxidation promoter, especially bromine, is the preferred mode of operation. However, for the best oxidation of pseudocumene and for the decrease of co-production of methyl-substituted phthalic acids as impurities, the manganese component of catalysis is either not added at the start of the reaction when all of the cobalt is added and then manganese's addition is delayed until the oxidation is 50 to 55 percent complete (e.g., 2 to 2.5 moles oxygen consumed per mole of pseudocumene), or a portion of the manganese is added with all of the cobalt and the remainder of the manganese is added stepwise in one or two series-connected oxidation zones. Such scheduled or delayed additions of manganese are reported in U.S. Pat. Nos. 3,491,144 and 3,683,016. It will be noted that the use of cerium as third catalyst has some benefit and it can be added at the beginning or scheduled to be added with the manganese.
Thus to reuse cobalt metal oxidation catalyst, that is, recycle it to the oxidation of pseudocumene, it is necessary to separate it or most of it from manganese after recovering cobalt and manganese from the trimellitic acid process residue.